The present invention relates to a multilayer product comprising four layers, the second and third layers being a layer composite consisting of a thermoplastic and a UV-stabilised polycarbonate, the second layer being a UV protection layer based on alkyl acrylate and the first layer being a scratch protection layer, which likewise contains UV absorber, the first and second layers having particular extinction values and the fourth layer likewise consisting of polycarbonate. The invention furthermore relates to the manufacture of these multilayer products and to products, for example glazing, which contain the said multilayer products.
Shaped bodies made of polycarbonate have already been known for a long time. Polycarbonate, however, has the disadvantage that it is not per se inherently UV-stable. The sensitivity curve of bisphenol A polycarbonate has the highest sensitivity between 320 nm and 330 nm. Below 300 nm no solar radiation reaches the Earth, and above 350 nm this polycarbonate is so insensitive that yellowing no longer takes place.
In order to protect polycarbonate against the harmful effect of UV rays in the atmosphere, UV stabilisers are generally used which absorb the UV radiation and convert it into non-harmful heat energy.
For durable protection, it is advantageous for the harmful UV radiation to be filtered out effectively before it reaches the polycarbonate surface, as may be done by using UV protection layers, for example coextrusion layers containing UV absorber, films containing UV absorber or coating compositions containing UV absorber, on polycarbonate.
Typical UV absorber classes, which are known to be usable for this, are 2-hydroxybenzophenones, 2-(2-hydroxyphenyl)benzotriazoles, 2-(2-hydroxyphenyl)-1,3,5-triazines, 2-cyanoacrylates and oxalanilides.
The prior art of multilayer products will be summarised below by way of example.
EP-A 0 110 221 discloses plates consisting of two polycarbonate layers, one layer containing at least 3 wt. % of a UV absorber. According to EP-A 0 110 221, these plates may be manufactured by coextrusion.
EP-A 0 320 632 discloses shaped bodies consisting of two layers of thermoplastic, preferably polycarbonate, one layer containing specially substituted benzotriazoles as UV absorbers. EP-A 0 320 632 also discloses the manufacture of these shaped bodies by coextrusion.
EP-A 0 247 480 discloses multilayer plates which contain a layer of branched polycarbonate in addition to a layer of thermoplastic, the polycarbonate layer containing specially substituted benzotriazoles as UV absorbers. The manufacture of these plates by coextrusion is likewise disclosed.
EP-A 0 500 496 discloses polymer compositions which are stabilised against UV light by special triazines, and their use as an outer layer in multilayer systems. Polycarbonate, polyester, polyamides, polyacetals, polyphenylene oxide and polyphenylene sulfide are mentioned as polymers.
US 2006/0234061 A1 discloses a multilayer product comprising a first layer and a second layer, the second layer containing polycarbonate and the first layer being a UV protection layer based on alkyl acrylate, which contains a diphenyl-substituted triazine as UV stabiliser.
WO 2006/121484 A2 discloses a plastic plate consisting of a polymer layer, a weather-resistant film layer and an abrasion protection layer, and the production of the overall composite. Polycarbonate, polymethyl methacrylate, polyester, polycarbonate/acrylonitrile butadiene styrene blend and polycarbonate/polyester blend are mentioned as polymers.
It has, however, been found that the known UV-stabilised polycarbonate shaped bodies still have insufficient long-term stability against yellowing for many applications, in particular for outdoor applications with constantly high requirements in relation to visual impression, for example glazing. The number of coating steps should furthermore be kept as small as possible, since they are expensive and susceptible to defects.
For such applications, it is required that a polycarbonate shaped body does not yellow by more than Δ YI 3 under 30 MJ/m2 of irradiation at 340 nm (corresponding to 10 years of outdoor weathering in Florida).
The weathering for this test is carried out in an Atlas Ci 5000 weatherometer with an irradiance of 0.75 W/m2/nm at 340 nm and a dry/rain cycle of 102:18 minutes. The black panel temperature is 70° C., the sample room temperature 55° C. and the relative humidity 40%.
It is an object of the present invention to provide polycarbonate shaped bodies which yellow as little as possible under the said weathering conditions (YI or ΔYI as low as possible). As few coating steps as possible should furthermore be required for reasons of processing time reduction and the susceptibility to coating defects.